patch 8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes #3652)

src/alloc.h

@@ -21,5 +21,13 @@ typedef enum {
 	aid_tagstack_items,
 	aid_tagstack_from,
 	aid_tagstack_details,
+	aid_sign_getdefined,
+	aid_sign_getplaced,
+	aid_sign_define_by_name,
+	aid_sign_getlist,
+	aid_sign_getplaced_dict,
+	aid_sign_getplaced_list,
+	aid_insert_sign,
+	aid_sign_getinfo,
 	aid_last
 } alloc_id_T;